1. Field of the Invention
The present invention relates to an optical amplifying apparatus connected mutually through an optical transmission line, an optical transmission system and a method of adjusting an optical transmission loss thereof.
2. Description of the Related Art
Development of a long-distance optical transmission system has been advancing in recent years. The long-distance optical transmission system is realized by connecting a number of optical amplifying repeaters to a sequence in an optical transmission line. FIG. 9 shows one example of a wavelength division multiplexing optical transmission system related to the present invention. Line terminating equipments (LTEs) 501 and 502 are connected to each other through optical transmission lines 530 to 53n. In the optical transmission lines 530 to 53n, n of optical amplifying repeaters 511 to 51n is provided in multiple stages. The wavelength division multiplexing optical transmission system transmits a signal light while compensating for attenuation received by the optical transmission lines using the respective optical amplifying repeaters. The respective optical amplifying repeaters include optical amplifiers 521 to 52n and compensate for the attenuation by amplifying the signal light.
Next, with reference to FIG. 9, operations of the wavelength division multiplexing optical transmission system will be described. Signal lights λ1 to λm outputted from respective optical transmitters 541 to 54m in the line terminating equipment 501 are multiplexed by an optical multiplexer 55 and outputted after amplified by an optical amplifier 57. The signal lights λ1 to λm, which are transmitted and attenuated through the optical transmission line 530, are amplified and compensated by the optical amplifier 521 in the optical amplifying repeater 511. Thereafter, the signal lights λ1 to λm are repeatedly attenuated and compensated by the respective optical transmission lines and the respective optical amplifiers, and are finally inputted to the line terminating equipment 502 through the optical transmission line 53n. The inputted signal lights λ1 to λm are compensated for the last time by an optical amplifier 58 in the line terminating equipment 502 and received by respective optical receivers 551 to 55m after demultiplexed by an optical demultiplexer 56.
Here, it is known that the optical amplifier has a noise figure (NF) changing with respect to an input level of the signal light (for example, US 2003/0076578 and the like). From the viewpoint of transmission quality, in order to obtain a desired noise figure, it is required to set the input level of the signal light within a certain range. However, there are a wide variety of transmission distances and optical transmission lines between optical amplifying repeaters, and there are various optical transmission losses. Thus, the level of the signal light inputted to the optical amplifier is never maintained constant. Therefore, it is demanded to easily adjust the input level of the signal light inputted to the optical amplifier or the optical transmission loss.
Particularly, at the time of introduction of the optical transmission system, in order to adjust the input level to the respective optical amplifiers, it is required to output at least one wavelength of signal light from the preceding optical amplifier. Accordingly, the input level is adjusted by receiving the signal light. The adjustment of the input level is performed, for example, by inserting an optical attenuator into an input part of the optical amplifier and setting an attenuation level.
As described above, in the conventional optical transmission system, at the time of introduction of the system, it is required to actually transmit the signal light in order to adjust the input level to the optical amplifier or the optical transmission loss. Thus, there is a problem that the input level of the optical amplifier or the optical transmission loss cannot be adjusted unless equipment and preparations for transmitting the signal light are completed.